fix out-of-bounds read parsing AVC SPS in ParseSPS

[ubeddulla]

What problem does this PR solve?

Issue Number: N/A

Problem Summary:

AVCDecoderConfigurationRecord::Create copies the sequence header into a buffer that is not NUL-terminated (DEFINE_SMALL_ARRAY), then calls ParseSPS(buf.data() + 2, sps_length). ParseSPS takes a butil::StringPiece, so the bare const char* is turned into one through the implicit StringPiece(const char*) constructor, which runs strlen. When a publisher sends an AVC sequence header whose SPS body has no zero byte, that strlen walks off the end of the copied record and reads out of bounds. The path is reachable from untrusted RTMP video input.

What is changed and the side effects?

Changed:

Pass buf.substr(2, sps_length) so ParseSPS receives an explicitly sized view and no strlen runs. It is the same idiom already used a few lines below when pushing into sps_list. I also added a regression test that feeds a zero-free SPS body, which trips ASan on the old code and stays clean after the fix.

Side effects:

• Performance effects: none.

• Breaking backward compatibility: none. The reads inside ParseSPS were already bounded by sps_length, so valid sequence headers parse exactly as before.

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

[wasphin]

LGTM

[chenBright]

LGTM